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Math Help - Do not understand Work problems?

  1. #1
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    Do not understand Work problems?

    A spring has a natural length of 26 cm. If a 21 N force is required to keep it stretched to a length of 28 cm, how much work W is required to stretch it from 26 cm to 32 cm?

    I used hookes law, F=kx and found k as 525x. So I integrated 525x with the intervals [.028 to .032] it was wrong.

    If 8 J of work are needed to stretch a spring from 11 cm to 13 cm and 12 J are needed to stretch it from 13 cm to 15 cm, what is the natural length L of the spring?

    I do not understand how to do this problem.. I never learned how to do Work in high school so this is very new to me.
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  2. #2
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    Quote Originally Posted by elpermic View Post
    A spring has a natural length of 26 cm. If a 21 N force is required to keep it stretched to a length of 28 cm, how much work W is required to stretch it from 26 cm to 32 cm?

    I used hookes law, F=kx and found k as 525x. So I integrated 525x with the intervals [.028 to .032] it was wrong.
    Why did you integrate it? F = kx. You know F and you know x. So now solve for k to find the spring constant, then use W = Fd to solve the work required.

    Also the units for work = Newton meters = Nm
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  3. #3
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    I did that.. My F is 21N and my distance should be .06 meters. I got 1.26 and my answer is wrong. My midterm is on Fridya and I don't understtand work.
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  4. #4
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    Quote Originally Posted by elpermic View Post
    My F is 21N and my distance should be .06 meters.
    No! They are different extensions with different forces. Don't mix them up.

    You have to work it through slowly, there are two parts to the first question. First you have to find the spring constant with the first part of the information, then use this to solve for the work for the second part of information.

    First piece of information given: F = 21N, x = 0.02m, k=?

    Second piece of information: F = ?, k = solved from before, d = 0.06, W = ?

    Can you try to solve it using the two formulae now?
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  5. #5
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    Ok I got k as 1050.

    So then do I use the F=kx equation again and this time replace x with 0.06? Then I get my F as 63 J

    How do I integrate it then? Do I use W= Intregral of Fkx from .02 to .06?
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